The invention concerns a device for determining a filling level in a container, with a stillwell which has an entry opening and a vent opening, with an ultrasonic transducer arranged in the stillwell and with at least one pre-chamber. The pre-chamber has an inlet opening to the container and an outlet opening to the entry opening of the stillwell.
In order to determine the oil level in engines, ultrasonic sensors are predominantly used. These emit sound waves which can be measured at the interface between two media, e.g. gas and oil, wherein other fluids can also be measured instead of oil, reflected and received again by the ultrasonic sensor. From the detected run time of the sound waves, the filling level can then be determined in the container which has the device. In an engine which is in operation, in particular in a motor vehicle, the filling level of the oil is, however, only difficult to determine or even unable to be determined at all, because the oil is highly foamed and the sound waves are reflected on the small gas bubbles in the oil. The detected values are therefore very intensively dispersed and indicate an oil level which is too low.
For measurements which are able to be evaluated, however, a clear interface is necessary between the media of gas and oil. In order to counteract the problem of intensively dispersive values, damping arrangements, e.g. so-called stillwells, are associated with the ultrasonic sensors. Inside these stillwells, which surround the measurement section of the ultrasonic sensor, the fluid which is to be measured is calmed, and is only connected with the container through a small opening of the stillwell to the container. Through the small opening of the stillwell to the container, fewer gas bubbles arrive into the stillwell. The filling level in the stillwell corresponds here to the fluid level outside the stillwell.
Further developments of such a stillwell are shown by European Patent Application No. EP 2 133 670 A1, German Patent Application No. DE 10 2008 027 969 A1, and PCT Application Publication No. WO 2008/009277 A1. In these publications, a pre-volume chamber is associated with the stillwell. These pre-volume chambers serve to degas the fluid which is to be measured, e.g. foamed oil, so that a fluid which is relative free of bubbles flows in the measurement section of the stillwell.
However, tests show that the foamed oil flows through the known pre-volume chambers and chambers very quickly in an engine in the case of sudden jumps in rotation speed, e.g. from 3000 to 4000 or 5000 revolutions/minute, without there being a possibility for the foamed oil to degas in the pre-volume chamber. It therefore occurs that small gas bubbles are brought into the measurement section of the stillwell and impede the correct indication of the filling level. As these small gas bubbles can only escape slowly from the measurement section of the stillwell, the known sensors also do not detect the actual filling level in the container for a long time after the jumps in rotation speed, although a measurable filling level is present outside the stillwell.
The time in which the filling level is not correctly detected is designated as the dead time and potentially leads to the fault indication of a filling level which is too low in the container. This problem occurs in particular in the case of a dynamic driving mode over a lengthy period of time, because here, owing to the long dead times in which the actual filling level is not determined, a mean value calculation is no longer possible, which can consequently lead to greater damage through “incorrect action” due to the false fault indication.